Ion exchange membranes (IEM) are used in fuel cells as solid electrolytes. A membrane is located between the cathode and anode and transports protons formed near the catalyst at the hydrogen electrode to the oxygen electrode thereby allowing a current to be drawn from the cell. These membranes are particularly advantageous as they replace heated acidic liquid electrolytes such as phosphoric acid fuel cells which are very hazardous.
Ion exchange membranes are used in chloralkali applications to separate brine mixtures and form chlorine gas and sodium hydroxide. The membrane selectively transports the sodium ions across the membrane while rejecting the chloride ions. IEM's are also useful in the area of diffusion dialysis where for example, caustic solutions are stripped of their impurities. The membranes are also useful for pervaporation and vapor permeation separations due to their ability to transfer polar species at a faster rate than their ability to transfer non-polar species.
These membranes must have sufficient strength to be useful in their various applications. Often this need for increased strength requires the membranes to be made thicker which decreases their ionic conductance. For example, ion exchange membranes that are not reinforced such as those commercially available from E. I. DuPont de Nemours, Inc. and sold under the trademark Nation are inherently weak at small thicknesses (e.g., less than 0.050 mm) and must be reinforced with additional materials causing the final product to have increased thickness. Moreover, these materials cannot be reliably manufactured pinhole free.
U.S. Pat. No. 3,692,569 to Grot relates to the use of a coating of a copolymer of fluorinated ethylene and a sulfonyl-containing fluorinated vinyl monomer on a fluorocarbon polymer that was previously non-wettable. The fluorocarbon polymer may include tetrafluoroethylene polymers (not porous expanded PTFE). This coating provides a topical treatment to the surface so as to decrease the surface tension of the fluorocarbon polymer. U.S. Pat. No. 4,453,991 to Grot relates to a process for making a liquid composition of a perfluorinated polymer having sulfonic acid or sulfonate groups in a liquid medium that is contacted with a mixture of water and a second liquid such as a lower alcohol. The liquid made by the process may be used as a coating, a cast film, and as a repair for perfluorinated ion exchange films and membranes. Cast or coated products made with the liquid composition had thicknesses on the order of 5 mils (0.125 mm).
U.S. Pat. No. 4,902,308 to Mallouk, et al. relates to a film of porous expanded PTFE having surfaces, both exterior and internal surfaces adjacent to pores, coated with a metal salt of perfluoro-cation exchange polymer. The base film of porous, expanded PTFE had a thickness of between 1 mil and 6 mils (0.025-0.150 mm). The final composite product had a thickness of at least 1 mil (0.025 mm) and preferably had a thickness of between 1.7 and 3 mils (0.043-0.075 mm). The composite product was permeable to air and the air flow as measured by the Gurley densometer ASTM D726-58 was found to be between 12 and 22 seconds.
U.S. Pat. No. 4,954,388 to Mallouk, et al. relates to an abrasion-resistant, tear resistant, multi-layer composite membrane having a film of continuous perfluoro ion exchange polymer attached to a reinforcing fabric by means of an interlayer of porous expanded PTFE. A coating of a perfluoro ion exchange resin was present on at least a portion of the internal and external surfaces of the fabric and porous ePTFE. The composite membrane made in accordance with the teachings of this patent resulted in thicknesses of greater than 1 mil (0.025 mm) even when the interlayer of porous ePTFE had a thickness of less than 1 mil (0.025 mm).
U.S. Pat. No. 5,082,472 to Mallouk, et al. relates to a composite membrane of microporous film in laminar contact with a continuous perfluoro ion exchange resin layer wherein both layers have similar area dimensions. Surfaces of internal pores of ePTFE may be coated at least in part with perfluoro ion exchange resin coating or the pores in the microstructure may be filled or partially filled with resin. The membrane of ePTFE had a thickness of about 2 mils (0.050 mm) or less and the perfluoro ion exchange layer in its original state had a thickness of about 1 mil (0.025 mm). The ePTFE layer of this composite membrane imparted mechanical strength to the composite structure and the pores of the ePTFE were preferably essentially unfilled so as to not block the flow of fluids.
U.S. Pat. Nos. 5,094,895 and 5,183,545 to Branca, et al. relate to a composite porous liquid-permeable article having multiple layers of porous ePTFE bonded together and having interior and exterior surfaces coated with a perfluoro ion exchange polymer. This composite porous article is particularly useful as a diaphragm in electrolytic cells. The composite articles are described to be relatively thick, preferably between from 0.76 and 5 mm.
U.S. Pat. No. 4,341,615 to Bachot, et al. relates to a fluorinated resin base material treated with a copolymer of an unsaturated carboxylic acid and a non-ionic unsaturated monomer for use as a porous diaphragm in the electrolysis of alkaline metal chlorides. The fluorinated resin base material may be reinforced with fibers such as asbestos, glass, quartz, zirconia, carbon, polypropylene, polyethylene, and fluorinated polyhalovinylidene (col. 2, lines 13-17). Only 0.1 to 6 percent of the total pore volume of the support sheet is occupied by the carboxylic copolymer.
U.S. Pat. No. 4,604,170 to Mivake et al. relates to a multi-layered diaphragm for electrolysis comprising a porous layer of a fluorine-containing polymer having a thickness of from 0.03 to 0.4 mm with its interior and anode-side surface being hydrophilic and an ion exchange layer on its cathode surface with the ion exchange layer being thinner than the porous layer but of at least 0.005 mm and the total thickness of the diaphragm being from 0.035 to 0.50 mm.
U.S. Pat. No. 4,865,925 to Ludwig, et al. relates to a gas permeable electrode for electrochemical systems. The electrode includes a membrane located between and in contact with an anode and a cathode. The membrane, which may be made of expanded polytetrafluoroethylene, may be treated with an ion exchange material with the resulting membrane maintaining its permeability to gas. Membrane thicknesses are described to be between 1 and 10 mils, (0.025-0.25 mm), with thicknesses of less than 5 mils (0.125) to be desirable. Examples show that membrane thicknesses range from 15 to 21 mils.
Japanese Patent Application No. 62-240627 relates to a coated or an impregnated membrane formed with a perfluoro type ion exchange resin and porous PTFE film to form an integral unit. No water or surfactant were used in the manufacture of this membrane. The combination is accomplished by fusion or by coating and does not provide for permanent adhesion of the ion exchange resin to the inside surface of the PTFE film. The weight ratio of the perfluoro ion exchange resin to PTFE is described to be in the range of 3 to 90% with a preferable weight ratio of 10 to 30%.
Japanese Application No. 62-280230 and 62-280231 relate to a composite structure in which a scrim or open fabric is heat laminated and encapsulated between a continuous perfluoro ion exchange membrane and an ePTFE sheet thus imparting tear strength to the structure. The composite membrane was not used for ionic conduction.
Additional research has also been conducted on the use of perfluorosulfonic acid polymers with membranes of expanded porous polytetrafluoroethylene such as that described in Journal Electrochem. Soc., Vol. 132, No. 2, February 1985, p. 514-515. The perfluoro ion exchange material was in an ethanol based solvent without the presence of water or surfactant. Moreover, ultrasonic energy in the treatment of this membrane.
Heretofore and as represented by the references discussed above, there is a need for an integral ultra-thin strong ion exchange composite membrane, with long term chemical and mechanical stability that has a thickness before swelling of at most 1 mil (0.025 mm), with more than 90% of the pore volume of the membrane filled with a perfluoro ionomer to render it at least substantially occlusive and that is capable of swelling without deterioration of mechanical properties.